'Short Term 12,' which reflects director Destin Daniel Cretton's experiences working at a group home, started out as a film school short. Now it's one of the year's best-reviewed movies.

Just after graduating from Point Loma Nazarene University in San Diego, Destin Daniel Cretton took a job at a local short-term care facility for at-risk teenagers, hoping to save money to make short films on his own while he figured out what to do next. Little did he know that his experience at the group home would become the basis not only for his film-school thesis project but also, five years later, one of the best-reviewed movies of the year, "Short Term 12."

"It was pretty heavy. I just felt like I was rattled a lot when I went in there," recalled Cretton, now 34. "I didn't even realize how much it changed me until I was looking back at it, thinking of who I was when I stepped into there, how naive I was. Coming out of that place, I learned how complicated life is."

"Short Term 12," which premiered at the South by Southwest Film Festival in March and opened in Los Angeles on Friday, concerns a young woman named Grace (Brie Larson) who works in a group home. She is living with her co-worker Mason (John Gallagher Jr.), the two trying to keep their romantic relationship separate from their relationship at work.

When a troubled girl, Jayden (Kaitlyn Dever), comes to stay in the facility, Grace sees in her a reflection of her own troubled past, causing Grace to finally confront issues she has long left unresolved. (The title refers to the facility itself, the small housing unit where much of the action takes place.)

As Cretton began to work on expanding "Short Term 12" from a short into a feature script after graduating from film school at San Diego State University, he found himself restless. "When I started to write the feature, honestly, I was just bored of writing," he recalled in an interview alongside Larson.

He hit on the idea of switching the gender of the lead character from a young man, his own more obvious stand-in, to a young woman.

"It was just a huge challenge for me to write from a female perspective," Cretton said. "But it was also a wonderful excuse for me to try to understand my sisters a little more, my mom a little more, my girlfriend at the time."

The change has proved particularly fortuitous for Larson, who has been winning accolades for her performance, including best actress at Switzerland's recent Locarno Film Festival. "Thank god you were bored," she said.

Though "Short Term 12" is her first lead role, the 23-year-old has been acting since she was a child, including a regular part on TV's "United States of Tara," and recent supporting film work, including "Greenberg," "Scott Pilgrim vs. the World," "Rampart," "21 Jump Street" and the upcoming "Don Jon."

Cretton first met Larson via Skype while the actress was shooting a role in "The Spectacular Now." Even over a computer, he knew she was just what he was looking for.

"I could see her brain working in her eyes," he said. "And I think you see that on screen, there are so many thoughts going through her head when she's not doing anything."

"I read it and the initial feeling was that it was so right for me," said Larson of "Short Term 12." "There was something about this script that from the second I read it I just knew. I just knew what I was supposed to do, I knew that person. And it wasn't really me, but it was like the deepest part of me was Grace."

"I was completely moved by the script in a way that I'm not often by screenplays," said Gallagher, who shot "Short Term" between his work on seasons one and two of TV's "The Newsroom." "It managed to evoke emotions so effortlessly, without being overly descriptive or heavy-handed. I cried reading the script, and I don't know if that's ever happened to me before."

Cretton now lives in Los Angeles — the film was shot at a former short-term care facility near Sylmar — but he has mostly been traveling to festivals since the premiere of "Short Term 12."

Though both Cretton's "Short Term 12" short and his first feature film, "I Am Not a Hipster," played at the Sundance Film Festival, programmers rejected the full-length version of "Short Term 12" for the 2013 Utah gathering.

When SXSW film festival producer Janet Pierson first saw "Short Term 12" late last year, she did not know it had been rejected by another festival. Rather, she saw the movie on the recommendations of her programming team and the film's editor, Nat Sanders, known for his work with director Lynn Shelton ("Humpday," "Your Sister's Sister").

"What for me stuck out was the performances and the idea that it wasn't sentimental," Pierson said. "You think you've seen it before and maybe you think you don't care, but then it's so affecting and so beautifully done. It's just so satisfying that people are connecting with it."

The film won both the grand jury and audience awards at SXSW and went on to take an audience prize at the Los Angeles Film Festival.

"The film didn't get into Sundance, but at the same time wouldn't exist without Sundance having supported the short and Destin's first feature," said "Short Term 12" producer Asher Goldstein in an email. "This year it was incredible to be welcomed into the SXSW family, which was truly the most perfect place to premiere the film. Both festivals are so important to us and we feel so lucky to have been a part of both."